Add context to train station POIs

[davecraig]

This is very closely related to #307 though I don't see any iOS strings that would help us. The problem I saw is that walking past High Street train station in Glasgow, the POI is simply read out as High Street. That's confusing as there's nothing to suggest that it's a train station. The iOS strings do have a string for a similar case for bus stops:

<string name="osm_tag_bus_stop_named">"%s Bus Stop"</string>

but we need to do similar for trains/metros/underground etc. e.g. High Street Station